How Is Phosphine Prepared and Why Is It Important in Chemistry?
One of the most important chemical structure in the study of elements is Phosphine (PH3). This is a colourless gaseous substance discovered by ‘Philippe Gengembre’ in 1783 and is often confused with the study of Phosphene. Let us hence define what is Phosphine with a clear-cut idea about its structure and function with formulas for the same. We will also learn about the laboratory preparation of Phosphine and how noxious this substance is brief.
What Is Phosphine?
Present in the chemical group called ‘organophosphorus’, Phosphine (IUPAC name: Phosphane) is an element that is a highly toxic gas with flammable properties. A French chemist Philippe once generated a regular supply of heat over phosphorous (P4) with an aqueous solution of potassium carbonate; the result was a colourless gaseous substance. This highly unpleasant smell of gas was coined as ‘Phosphine’ due to its rotting fish odour. Phosphine is classified in the periodic table as in the pnictogen hydride section. Phosphine (PH3) is essential to the biochemical cycle, even though it possesses critical chemical properties with an unstable compound concentration in the atmosphere.
Structure of Phosphine
The entire molecular structure of Phosphine is a trigonal pyramid. The diagram of Phosphine is studies based on the Lewis structure. Lewis structure is also called electron dot structures and is a picture to represent the lone pair of electrons and bonds with atoms or molecules. This can be grasped quickly using the following pointers:
Phosphine lewis structure has 8 valence electrons.
Since both N (Nitrogen) and P (Phosphorous) are present in the same group in the periodic table, the structure of PH3 is similar to NH3 (Ammonia).
However, the electro-negativity of Phosphorus is lesser than that of Nitrogen.
93.5 is the bond angle between H-P-H regions in the structure of Phosphine.
The bond length in P-H is 1.42 A.
Phosphine is considered to be a Lewis base. This is due to the presence of a non-bonding electrons pair on the P shell which may be donated.
Preparation of Phosphine
The preparation of Phosphine gas begins by mixing dilute Hydrochloric Acid (HCl) or water with calcium phosphide (Ca3P2). The below formula represents this preparation.
Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3
Ca3P2 + 6HCl → 3CaCl2 + 2PH3
As you might have noticed, the other results along with Phosphine are Calcium Hydroxide (in the case of water) and Calcium Chloride (in the case of hydrochloric acid).
The laboratory preparation of Phosphine goes by the following case:
P4+3NaOH+3H2O → PH3 + 3NaH2PO2
Here, White Phosphorus has to be heated with concentrated Sodium Hydroxide to gove out Phosphine. Note that this preparation has to be undertaken in an inert atmosphere to make use of Carbon dioxide (CO2).
Physical and chemical properties of Phosphine
Physical properties of Phosphine are given below:
Formula - PH3
Critical Temp. (°F) - 124.9
Density of the gas @ 70°F 1 atm (lb/ft3) - 0.0885
Specific Gravity - 1.203
Melting Point (°F) - 208.8
Boiling Point (°F) - 126.0
Molecular Weight (lb/mol) - 34.00
Critical Pressure (Pc) - 947.9
Liquid Density @ 70°F (lb/ft3) - 35.50
Understand the chemical properties and functions of Phosphine gas with the following pointers in mind:
Exploding nature is high if disclosed to oxidising agents.
High dissolution property in certain solvents and sparingly dissolvable in water.
An extremely noxious gas.
According to chemical safety, Phosphine possesses the characteristic to cause high environmental damage.
Acts as a lewis base when its lone electron is donated in reaction with hydrogen iodide.
Accurately toxic and highly flammable.
Smells more like a spoiled fish or a clove of garlic.
When in contact with even minimal amount of Phosphine, it can trigger problems such as dizziness, jaundice, loose bowels, kidney and liver damager, inflammation of nasal cavity, fatigue, regurgitating, cerebral pain, convulsions, coma, shock and more.
Other names to denote PH3 - Phosphorus hydride, Fosforowodor, Phosphorwasserstoff, Trihydrogen phosphide, Gas-ex-B and Hydrogen phosphide.
Uses of Phosphine
Accounting to the uses of Phosphine, there are a few important roles that PH3 plays in different areas of work. The nature of combustion is spontaneous in PH3 and is hence useful as in the Holme’s signal for the same. Plastic industries make use of PH3 to create new flammable kits. Phosphine is a Dopant (substance useful to form ceratin electrical component) in semiconductor factories. This substance is also preferred in the preemption of incendiaries and flame retardants. In the process of fumigating grains and animal feed, Phosphene plays the key role.
FAQs on Phosphine: Structure, Properties, Preparation, and Uses
1. What is Phosphine and what is its chemical formula?
Phosphine is a covalent compound of phosphorus and hydrogen with the chemical formula PH₃. It is a colourless, highly poisonous gas that is a hydride of phosphorus. It is the phosphorus analogue of ammonia (NH₃) and is primarily known for its characteristic unpleasant smell, often described as being like garlic or rotten fish.
2. How is Phosphine (PH₃) prepared in the laboratory as per the NCERT syllabus for Class 12?
In the laboratory, Phosphine is prepared by heating white phosphorus with a concentrated sodium hydroxide (NaOH) solution in an inert atmosphere of carbon dioxide (CO₂). This reaction produces Phosphine gas and sodium hypophosphite as a byproduct. The balanced chemical equation for this preparation is:
P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂
3. What are the key physical and chemical properties of Phosphine?
Phosphine exhibits several distinct properties:
Physical Properties: It is a colourless gas with a strong, unpleasant smell of rotten fish. It is slightly soluble in water but soluble in organic solvents.
Chemical Properties: It is a weak base, reacting with acids like HBr to form phosphonium bromide (PH₄Br). Pure phosphine is not spontaneously flammable, but becomes so due to the presence of P₂H₄ or P₄ vapour impurities. It is also a powerful reducing agent.
4. Explain the structure and bonding in a Phosphine (PH₃) molecule.
The Phosphine (PH₃) molecule has a trigonal pyramidal structure, similar to ammonia. The central phosphorus atom is sp³ hybridised. Three of these hybrid orbitals form sigma bonds with three hydrogen atoms, while the fourth orbital contains a lone pair of electrons. Due to strong repulsion between the lone pair and bond pairs, the H-P-H bond angle is compressed to approximately 93.6°, which is much smaller than the standard tetrahedral angle of 109.5°.
5. Why is Phosphine a much weaker base than Ammonia (NH₃), despite both having a lone pair of electrons?
Phosphine is a significantly weaker base than ammonia because the lone pair of electrons on the phosphorus atom is less available for donation. In PH₃, the lone pair occupies a larger and more diffuse 3s orbital, making it less concentrated and harder to donate. In contrast, the lone pair on nitrogen in NH₃ is in a smaller, more compact 2s orbital, making it more readily available to accept a proton. This difference in electron availability explains the lower basicity of phosphine.
6. What happens when Phosphine is bubbled through a solution of copper sulphate?
When Phosphine gas is passed through a solution of copper sulphate (CuSO₄), it demonstrates its strong reducing properties. It reduces the copper ions (Cu²⁺) and forms a black precipitate of copper phosphide (Cu₃P₂). The corresponding reaction is:
3CuSO₄ + 2PH₃ → Cu₃P₂ + 3H₂SO₄
7. What are the most important uses of Phosphine in industries and technology?
Phosphine has several critical applications, primarily due to its high reactivity and toxicity. Its main uses include:
Fumigant: It is widely used as a fumigant pesticide to protect stored cereal grains and other agricultural products from pests.
Holme's Signals: It is used to produce smoke screens and distress signals for ships at sea, which ignite spontaneously upon contact with air.
Semiconductor Industry: It is used as a source of phosphorus to introduce it as a dopant in silicon crystals for manufacturing n-type semiconductors.
8. Why is spontaneously flammable phosphine used in Holme's signals?
Holme's signals utilise the spontaneous flammability of impure phosphine for creating smoke signals. Containers of calcium carbide and calcium phosphide are pierced and thrown into the sea. They react with water to produce acetylene and phosphine gas, respectively. The phosphine produced is impure, containing traces of diphosphine (P₂H₄), which is spontaneously flammable in air. This impurity ignites the phosphine and acetylene, producing a large, visible flame and a thick smoke screen, which serves as a distress signal.
9. Is Phosphine gas toxic to humans? Explain its effects.
Yes, Phosphine gas is extremely toxic to humans and other aerobic life forms. Inhalation is the primary route of exposure. It acts as a metabolic poison by disrupting cellular respiration. Exposure can cause severe health effects, including:
Severe irritation of the respiratory tract, leading to pulmonary oedema.
Depression of the central nervous system.
Damage to vital organs like the liver, kidneys, and heart.
Symptoms like dizziness, fatigue, nausea, and severe gastrointestinal pain.
10. Why must the laboratory preparation of phosphine be conducted under an inert atmosphere of CO₂?
The laboratory preparation of phosphine must be done in an inert atmosphere of CO₂ to prevent a potential explosion. The phosphine gas produced during the reaction is often contaminated with small amounts of diphosphine (P₂H₄) vapour. This impurity is highly reactive and catches fire spontaneously upon contact with oxygen in the air. By flushing the apparatus with an inert gas like CO₂, oxygen is displaced, ensuring that the flammable phosphine mixture does not ignite, making the procedure safe.
